In recent years, computerization of information within companies are progressing, and it is a problem of an IT system to share massive files stored in each site between sites distributed geographically as fast as possible. As one method of solving the problem, there is a multi-site file system sharing via a data center, in which file servers of the sites share an identical file system, via the data center connected by a WAN (Wide Area Network).
For example, Patent Literature 1 discloses a system of enabling a file system in a NAS (Network Attached Storage) in one site to be referred as a read-only file system from NAS in other site, through a CAS (Content-Addressable Storage) device of a data center. In the present system, the original site performs a synchronization process to synchronize update contents periodically (for example, once every day) between the site and the data center. The other sites referring the original site offer the updated contents, by periodically reflecting the updated content in the data center to own site.
In the case where an update access is permitted to a plurality of sites in the multi-site file system sharing via the data center, there is a fear that the updates of differing contents collide (conflict) with respect to an identical file between the sites. In the case where a conflict happens, a conflict dissolving process which compares the update contents and decides which update should be remained as a newest version becomes necessary. For example, in Patent Literature 2, a method of performing the conflict dissolving at the time of data synchronization of the file system is disclosed.
Further, as a method for realizing exclusive control of the update process among a plurality of the sites, Patent Literature 3 discloses a system of providing exclusive control for a name space of a file system in sub tree granularities. The name space used herein is a managed storage object to manage files within the file system, and generally is a tree structure taking the directory as a node and a file as a leaf. Further, sub tree becomes a part of the tree structure of the file system.